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          <th colspan="3" align="center">Queue and Recno access method specific configuration</th>
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		Access Method Configuration
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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="rq_conf"></a>Queue and Recno access method specific configuration</h2>
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        </div>
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      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="rq_conf.html#am_conf_recno">Managing record-based databases</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="rq_conf.html#am_conf_extentsize">Selecting a Queue extent size</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="rq_conf.html#am_conf_re_source">Flat-text backing files</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="rq_conf.html#am_conf_renumber">Logically renumbering records</a>
            </span>
          </dt>
        </dl>
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      <p>
    There are a series of configuration tasks which you can perform when
    using the Queue and Recno access methods.  They are described in the following sections.
</p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="am_conf_recno"></a>Managing record-based databases</h3>
            </div>
          </div>
        </div>
        <p>When using fixed- or variable-length record-based databases, particularly
with flat-text backing files, there are several items that the user can
control.  The Recno access method can be used to store either variable-
or fixed-length data items.  By default, the Recno access method stores
variable-length data items.  The Queue access method can only store
fixed-length data items.</p>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp50888032"></a>Record Delimiters</h4>
              </div>
            </div>
          </div>
          <p>When using the Recno access method to store variable-length records,
records read from any backing source file are separated by a specific
byte value which marks the end of one record and the beginning of the
next.  This delimiting value is ignored except when reading records from
a backing source file, that is, records may be stored into the database
that include the delimiter byte.  However, if such records are written
out to the backing source file and the backing source file is
subsequently read into a database, the records will be split where
delimiting bytes were found.</p>
          <p>For example, UNIX text files can usually be interpreted as a sequence of
variable-length records separated by ASCII newline characters.  This byte
value (ASCII 0x0a) is the default delimiter.  Applications may specify a
different delimiting byte using the <a href="../api_reference/C/dbset_re_delim.html" class="olink">DB-&gt;set_re_delim()</a> method.  If no
backing source file is being used, there is no reason to set the
delimiting byte value.</p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp50880416"></a>Record Length</h4>
              </div>
            </div>
          </div>
          <p>When using the Recno or Queue access methods to store fixed-length
records, the record length must be specified.  Since the Queue access
method always uses fixed-length records, the user must always set the
record length prior to creating the database.  Setting the record length
is what causes the Recno access method to store fixed-length, not
variable-length, records.</p>
          <p>The length of the records is specified by calling the
<a href="../api_reference/C/dbset_re_len.html" class="olink">DB-&gt;set_re_len()</a> method.  The default length of the records is 0 bytes.
Any record read from a backing source file or otherwise stored in the
database that is shorter than the declared length will automatically be
padded as described for the <a href="../api_reference/C/dbset_re_pad.html" class="olink">DB-&gt;set_re_pad()</a> method.  Any record stored
that is longer than the declared length results in an error.  For
further information on backing source files, see
<a class="xref" href="rq_conf.html#am_conf_re_source" title="Flat-text backing files">Flat-text backing files</a>.</p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp50867536"></a>Record Padding Byte Value</h4>
              </div>
            </div>
          </div>
          <p>When storing fixed-length records in a Queue or Recno database, a pad
character may be specified by calling the <a href="../api_reference/C/dbset_re_pad.html" class="olink">DB-&gt;set_re_pad()</a> method.  Any
record read from the backing source file or otherwise stored in the
database that is shorter than the expected length will automatically be
padded with this byte value.  If fixed-length records are specified but
no pad value is specified, a space character (0x20 in the ASCII
character set) will be used.  For further information on backing source
files, see <a class="xref" href="rq_conf.html#am_conf_re_source" title="Flat-text backing files">Flat-text backing files</a>.</p>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="am_conf_extentsize"></a>Selecting a Queue extent size</h3>
            </div>
          </div>
        </div>
        <p>
        In Queue databases, records are allocated sequentially and directly
        mapped to an offset within the file storage for the database.  As
        records are deleted from the Queue, pages will become empty and will
        not be reused in normal queue operations.  To facilitate the
        reclamation of disk space a Queue may be partitioned into extents.
        Each extent is kept in a separate physical file.
    </p>
        <p>
        Extent files are automatically created as needed and marked for
        deletion when the head of the queue moves off the extent.  The extent
        will not be deleted until all processes close the extent.  In addition,
        Berkeley DB caches a small number of extents that have been recently
        used; this may delay when an extent will be deleted. The number of
        extents left open depends on queue activity.
    </p>
        <p>
        The extent size specifies the number of pages that make up each extent.
        By default, if no extent size is specified, the Queue resides in a
        single file and disk space is not reclaimed.  In choosing an extent
        size there is a tradeoff between the amount of disk space used and the
        overhead of creating and deleting files.  If the extent size is too
        small, the system will pay a performance penalty, creating and deleting
        files frequently.  In addition, if the active part of the queue spans
        many files, all those files will need to be open at the same time,
        consuming system and process file resources.
    </p>
        <p>
        You can set the Queue extent size using the <a href="../api_reference/C/dbset_q_extentsize.html" class="olink">DB-&gt;set_q_extentsize()</a>
        method. You can see the current extent size using the
        <a href="../api_reference/C/dbget_q_extentsize.html" class="olink">DB-&gt;get_q_extentsize()</a> method.
    </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="am_conf_re_source"></a>Flat-text backing files</h3>
            </div>
          </div>
        </div>
        <p>It is possible to back any Recno database (either fixed or variable
length) with a flat-text source file.  This provides fast read (and
potentially write) access to databases that are normally created and
stored as flat-text files.  The backing source file may be specified by
calling the <a href="../api_reference/C/dbset_re_source.html" class="olink">DB-&gt;set_re_source()</a> method.</p>
        <p>The backing source file will be read to initialize the database.  In the
case of variable length records, the records are assumed to be separated
as described for the <a href="../api_reference/C/dbset_re_delim.html" class="olink">DB-&gt;set_re_delim()</a> method.  For example, standard
UNIX byte stream files can be interpreted as a sequence of variable
length records separated by ASCII newline characters.  This is the
default.</p>
        <p>When cached data would normally be written back to the underlying
database file (for example, when the <a href="../api_reference/C/dbclose.html" class="olink">DB-&gt;close()</a> or
<a href="../api_reference/C/dbsync.html" class="olink">DB-&gt;sync()</a> methods are called), the in-memory copy of the database will
be written back to the backing source file.</p>
        <p>The backing source file must already exist (but may be zero-length) when
<a href="../api_reference/C/dbopen.html" class="olink">DB-&gt;open()</a> is called.  By default, the backing source file is read
lazily, that is, records are not read from the backing source file until
they are requested by the application. If multiple processes (not
threads) are accessing a Recno database concurrently and either
inserting or deleting records, the backing source file must be read in
its entirety before more than a single process accesses the database,
and only that process should specify the backing source file as part of
the <a href="../api_reference/C/dbopen.html" class="olink">DB-&gt;open()</a> call.  This can be accomplished by calling the
<a href="../api_reference/C/dbset_flags.html" class="olink">DB-&gt;set_flags()</a> method with the <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_SNAPSHOT" class="olink">DB_SNAPSHOT</a> flag.</p>
        <p>Reading and writing the backing source file cannot be transactionally
protected because it involves filesystem operations that are not part of
the Berkeley DB transaction methodology. For this reason, if a temporary
database is used to hold the records (a NULL was specified as the file
argument to <a href="../api_reference/C/dbopen.html" class="olink">DB-&gt;open()</a>), <span class="bold"><strong>it is possible to lose the
contents of the backing source file if the system crashes at the right
instant</strong></span>. If a permanent file is used to hold the database (a filename
was specified as the file argument to <a href="../api_reference/C/dbopen.html" class="olink">DB-&gt;open()</a>), normal database
recovery on that file can be used to prevent information loss.  It is
still possible that the contents of the backing source file itself will
be corrupted or lost if the system crashes.</p>
        <p>For all of the above reasons, the backing source file is generally used
to specify databases that are read-only for Berkeley DB applications, and that
are either generated on the fly by software tools, or modified using a
different mechanism such as a text editor.</p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="am_conf_renumber"></a>Logically renumbering records</h3>
            </div>
          </div>
        </div>
        <p>Records stored in the Queue and Recno access methods are accessed by
logical record number.  In all cases in Btree databases, and optionally
in Recno databases (see the <a href="../api_reference/C/dbset_flags.html" class="olink">DB-&gt;set_flags()</a> method and the
<a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_RENUMBER" class="olink">DB_RENUMBER</a> flag for more information), record numbers are
mutable.  This means that the record numbers may change as records are
added to and deleted from the database.  The deletion of record number
4 causes any records numbered 5 and higher to be renumbered downward by
1;  the addition of a new record after record number 4 causes any
records numbered 5 and higher to be renumbered upward by 1.  In all
cases in Queue databases, and by default in Recno databases, record
numbers are not mutable, and the addition or deletion of records to the
database will not cause already-existing record numbers to change.  For
this reason, new records cannot be inserted between already-existing
records in databases with immutable record numbers.</p>
        <p>Cursors pointing into a Btree database or a Recno database with mutable
record numbers maintain a reference to a specific record, rather than
a record number, that is, the record they reference does not change as
other records are added or deleted.  For example, if a database contains
three records with the record numbers 1, 2, and 3, and the data items
"A", "B", and "C", respectively, the deletion of record number 2 ("B")
will cause the record "C" to be renumbered downward to record number 2.
A cursor positioned at record number 3 ("C") will be adjusted and
continue to point to "C" after the deletion.  Similarly, a cursor
previously referring to the now deleted record number 2 will be
positioned between the new record numbers 1 and 2, and an insertion
using that cursor will appear between those records.   In this manner
records can be added and deleted to a database without disrupting the
sequential traversal of the database by a cursor.</p>
        <p>Only cursors created using a single <a href="../api_reference/C/db.html" class="olink">DB</a> handle can adjust each
other's position in this way, however.  If multiple <a href="../api_reference/C/db.html" class="olink">DB</a> handles
have a renumbering Recno database open simultaneously (as when multiple
processes share a single database environment), a record referred to by
one cursor could change underfoot if a cursor created using another
<a href="../api_reference/C/db.html" class="olink">DB</a> handle inserts or deletes records into the database.  For
this reason, applications using Recno databases with mutable record
numbers will usually make all accesses to the database using a single
<a href="../api_reference/C/db.html" class="olink">DB</a> handle and cursors created from that handle, or will
otherwise single-thread access to the database, for example, by using
the Berkeley DB Concurrent Data Store product.</p>
        <p>In any Queue or Recno databases, creating new records will cause the
creation of multiple records if the record number being created is more
than one greater than the largest record currently in the database. For
example, creating record number 28, when record 25 was previously the
last record in the database, will implicitly create records 26 and 27
as well as 28.  All first, last, next and previous cursor operations
will automatically skip over these implicitly created records.  So, if
record number 5 is the only record the application has created,
implicitly creating records 1 through 4, the <a href="../api_reference/C/dbcget.html" class="olink">DBC-&gt;get()</a> method with the
<a href="../api_reference/C/dbcget.html#dbcget_DB_FIRST" class="olink">DB_FIRST</a> flag will return record number 5, not record number 1.
Attempts to explicitly retrieve implicitly created records by their
record number will result in a special error return,
<a class="link" href="program_errorret.html#program_errorret.DB_KEYEMPTY">DB_KEYEMPTY</a>.</p>
        <p>In any Berkeley DB database, attempting to retrieve a deleted record, using
a cursor positioned on the record, results in a special error return,
<a class="link" href="program_errorret.html#program_errorret.DB_KEYEMPTY">DB_KEYEMPTY</a>.  In addition, when using Queue databases or Recno
databases with immutable record numbers, attempting to retrieve a deleted
record by its record number will also result in the <a class="link" href="program_errorret.html#program_errorret.DB_KEYEMPTY">DB_KEYEMPTY</a>
return.</p>
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